1. Field of the Invention
This invention relates to a coating method for a metallic surface for applying a thin film on the surface of metal by surface processing, such as plating. More particularly, it relates to a coating method for a metallic surface for applying a thin film mainly composed of an amorphous-like fluorine resin.
2. Description of the Related Art
In a metallic object, employing a metallic material, it needs to be improved in wear resistance, corrosion resistance and resistance against chemicals, depending on the objective of use. That is, metallic objects are improved in wear resistance, corrosion resistance and resistance against chemicals by being subjected to various surface processing operations.
Specifically, metal surfaces are modified by, for example, anodic oxidation processing, chemical conversion coating and plating processing. In particular, if Al, Mg, Ti or an alloy thereof is used, anodic oxidation processing or chemical processing is executed using an electrolytic solution mainly composed of acid, whereas, if Fe or other metal is used, a variety of plating processing operations are executed by electrical or chemical methods.
However, in a surface processed film, obtained by this surface processing, there exist gaps, such a fine pores, holes, crevices or micro-irregularities. For example, in an alumite film, obtained on anodic oxidation processing of an Al alloy, for example, fine pores not larger than 100 nm are produced in the cell in the course of the alumite film growth process. Also, in a surface processed film of Al or Mg alloys, random holes and micro-irregularities are produced by so-called break-down. In the plating processing or chemical conversion coating, crevices or cracks are formed due to stress in generating surface processed films. Specifically, in a Cr plated film or in a chromated film, obtained on plating processing or on chemical conversion coating, crevices or holes of approximately 1 xcexcm or less in size are inevitably produced.
Thus, the above-described surface processing cannot be said to be sufficient in improving wear resistance, corrosion resistance and resistance against chemicals. Moreover, the surface processed as described above is affected by foreign matter intruded and deposited in the fine holes or crevices present in the processed surface.
For example, in a metal mold used for molding a semiconductor device, resins tend to be affixed to the cavity surface to deteriorate resin releasing properties. Also, in a mold used for bending pins of lead frames, it is a frequent occurrence that Pb plating films on the lead frame surface become affixed to the mold to cause changes in the bending angle of the pins of the lead frames. Moreover, in a metal mold used for molding the rubber or molding synthetic resin vessels, it is a frequent occurrence that molding materials are affixed to the cavity surface to produce molded products unsatisfactory in shape or in luster.
This is thought to be presumably due to holes or crevices formed in the metal surface constituting the cavity.
Moreover, the water using sites in homes, such as the back sides of city water taps, tend to be contaminated inevitably to produce a non-hygienic state. On the other hand, a rotor of a centrifuging machine used for blood inspection is washed for disinfection and hence tends to be attacked if the rotor is coated only with an alumite film. It is felt that the fine holes or crevices formed in surface processed films account for contamination or infection.
For solving the problems attributable to the fine holes or crevices formed in surface processed films of a metal article, it has been proposed to form a crystalline fluorocarbon resin, such as polytetrafluoroethylene (PTFE) on a surface processed film of a metal article. There is, for example, a method of forming a crystalline fluorocarbon resin, such as polytetrafluoroethylene (PTFE), on an alumite processed Al surface. Specifically, the pores or holes in the alumite film may be stopped by impregnating PTFE two to several xcexcm in diameter dispersed in lukewarm water, as disclosed in Japanese Patent Publication S39-6113 1.
There is also such a technique in which a Cr plated film presenting fine holes or crevices is etched by a reverse current supplying method to enlarge the crevices or holes to a width of 3 to 10 xcexcm and in which the film so processed is impregnated with PTEF in an oven heated to a temperature of approximately 200xc2x0 C. at a pre-set pressure and cooled down. Since this can enlarge the diameter of the crevices and holes to approximately 3 to 10 xcexcm, it is possible to impregnate the inside of the crevices or holes having the diameter of the order of a few xcexc/m.
However, if an alumite film is formed by anodic oxidation processing on the surface of a metal article employing Al or Al alloys, ultra-fine holes or crevices are produced in the alumite film. If, in such case, the holes or crevices are increased in width by a technique such as the reverse current supplying method, the holes or crevices formed in the alumite film can be enlarged only to the size of the order of 100 mn. Therefore, if a thin film is formed with PTEF with the particle size of the order of a few xcexcm, the fine holes or crevices on the alumite film surface cannot be hidden such that simply PTFE particles ride on the fine holes or crevices.
Thus, if a crystalline fluorine-based resin film is formed on an alumite film, it is peeled off in a shorter time such that superior surface properties cannot be maintained for prolonged time.
On the other hand, a Cr plated film can be etched by the reverse current supplying method so that PTEF or the line crystalline fluorine resin can be formed to the inner recesses in the holes or crevices provided that the holes or the crevices are enlarged in diameter, as discussed above. However, in such case, the Cr plated film needs to be of a film thickness of at least 30 xcexcm. If, with the Cr plated film having a film thickness not larger than 30 xcexcm, the holes or crevices are increased in diameter, there are occasion wherein these holes or crevices reach the lower layer of a matrix material.
Thus, with the Cr plated film, the matrix material may be corroded due to the holes or crevices, or the Cr plated film itself tends to be peeled off.
It is therefore an object of the present invention to provide a coating method for a metal substrate whereby a coating film can be foiled reliably and strongly on a surface processed film formed on a metal matrix material, and a metal material obtained in this manner.
In one aspect, the present invention provides a method for coating a metal substrate including an immersion step of immersing a surface-processed metal substrate in a solution mainly composed of an amorphous-like fluorine resin, a polymerization step of polymerizing the amorphous-like fluorine resin and the surface processed metal substrate, and a step of forming a coating film on a surface-processed surface of the metal substrate.
In the present coating method for a metal substrate, according to the present invention, the amorphous-like fluorine resin is polymerized on the surface-processed surface. With this technique, since the amorphous-like fluorine resin is used, this amorphous-like fluorine resin is intruded into fine holes or crevices to constitute a coating film.
In another aspect, the present invention provides a metallic material including a metal substrate made up of a metal matrix material and a surface processed film, layered thereon, and a coating film obtained on polymerizing an amorphous-like fluorine resin on a surface of the metal substrate. The surface processed film is obtained on surface processing the metal matrix material,
In the metallic material of the present invention, the amorphous-like fluorine resin is polymerized to the surface of the metal substrate, that is to the major surface of the surface processed film obtained on surface processing. In the present metallic material, the amorphous-like fluorine resin can be intruded into fine holes and crevices formed in the metal substrate surface, so that the metallic material has a strong coating film insusceptible to peeling.
With the metal material coating method according to the present invention, in which the amorphous-like fluorine resin is polymerized on a surface-processed metal substrate, coating can be achieved reliably. Thus, with this technique, a coating film can be formed reliably even on a surface presenting extremely fine holes or crevices.
With the metal material of the present invention, the coating film polymerized on the surface-processed metal substrate is mainly composed of the amorphous-like fluorine resin. Thus, the present metal material is superior in intimate bonding between the metal substrate and the coating film, so that the meritorious effect of improving surface properties by the coating film can be maintained for prolonged time. Therefore, the present metal material can be used with advantage for various types of metallic products.